The UK is under threat of cyber attack, the national security strategy says. Photograph: Steve Caplin/Guardian

The government today unveiled its national security strategy, which emphasised the new threats facing Britain from cyberspace, non-state groups and hostile forces unlikely to be deterred by conventional weaponry.

Coming before Wednesday's defence review on military spending, the white paper identified four "tier one" threats to Britain: terrorism, attacks in cyberspace, a large accident or hazard and a military crisis – the only one relating to a conventional emergency that would involve other states.

The defence review will confirm that the navy will get two aircraft carriers at a total cost of £5bn, by far the most expensive single item in the armed forces' inventory, although initially there will be no aircraft available to go on them.

The RAF's ageing Tornado jets will be reprieved, at least in the medium term.

The review will also allow extra resources for countering cyber attacks, amounting to about £500m. GCHQ, the codebreaking and eavesdropping centre, is likely to benefit.

Officials did not say whether the government would develop an offensive cyber warfare capability as the US has done.

But they made it clear that MI5 and MI6, which have enjoyed budget increases in recent years, would not escape an austerity package in the spending review.

The tier two list of threats includes potential attacks on the UK by other states employing chemical, biological, radiological or nuclear weapons, insurgency overseas and attacks on satellites.

Tier three threats include disruption to gas and oil supplies, a rise in the "level of terrorists, organised criminals, illegal immigrants, and illicit goods trying to cross the UK border to enter the UK", and attacks on a UK overseas territory "as the result of a sovereignty dispute".

The white paper was based on risk assessments taking into account the impact of a potential attack and its likelihood, senior British officials said.

The paper states: "At present, only al-Qaida represents a major ideologically-driven threat to the national security of the UK and our allies.

"But … some regionally based ideologies could affect us through our role as an international hub, through the engagement of some [in] diaspora populations, or through driving conflict which impacts on our interests.

"It is a realistic possibility that, in the next 10 years, extremists … could cross the line between advocacy and terrorism."

The white paper was welcomed by anti-nuclear groups including Greenpeace, which claimed the MoD had wasted money "equipping itself to fight the last war rather than the next".

It added: "Yet that doesn't seem to be deterring the government from … plans to waste £97bn on Trident's replacement, a cold war weapon entirely irrelevant to the key threats."

The shadow foreign secretary, Yvette Cooper, said: "Our concern is, this [was] produced to provide cover for what the defence secretary admits has been a rushed defence review, not strategic at all."